DOC: document relations between internal entities

Links between internal structures have been represented in
entities.fig. The pdf and svg versions were added too.
diff --git a/doc/internals/entities.fig b/doc/internals/entities.fig
new file mode 100644
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--- /dev/null
+++ b/doc/internals/entities.fig
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+#FIG 3.2
+Portrait
+Center
+Metric
+A4      
+100.00
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+-6